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We critically review the recent literature on six EEG and MEG markers of the neural correlates of consciousness (NCC) for visual, auditory and tactile stimuli in neurotypical volunteers and neurological patients. After ruling out four of these as candidate NCC, we focus on two prominent evoked signals: an early, modality-specific negativity, termed the visual or auditory awareness negativity (VAN and AAN, respectively) and a late, modality-independent positivity termed the P3b. More than twelve diverse experimental studies found that the P3b is absent despite consciously seeing, hearing, or feeling stimuli, ruling out the P3b as a true NCC. In contrast, no convincing evidence for a dissociation between the awareness negativities and consciousness has been reported thus far. Furthermore, there is evidence for an equivalent signal in the tactile domain, which we term the somatosensory awareness negativity (SAN). These three neural signals are usually maximal on the side of the scalp contralateral to the evoking stimulus, above the associated sensory cortices. We conclude that the data from these three modalities is consistent with a generalized awareness negativity (GAN) correlated with perceptual consciousness that arises 120-200 ms following stimulus onset and originates from the underlying sensory cortices. The identification of this GAN points towards new, promising avenues for future research and raises an array of concrete questions that can be empirically investigated.

What are the neural processes associated with perceptual awareness that are distinct from pre-conscious sensory encoding and post-perceptual processes such as reporting an experience? Here, we used electroencephalography (EEG) and a no-report visual masking paradigm with parametric manipulations of stimulus visibility to search for neural signatures associated with perceptual awareness independent from both early sensory processing and subsequent reporting. Overall, we found only one neural signal that was uniquely associated with perceptual awareness: a fronto-central event-related potential (ERP) that we refer to as the N2. In contrast, earlier ERP signals were linked with the linear stimulus manipulation, while later candidate signatures, such as P3b and temporal generalization of decoding, were present in the report condition but disappeared in the no-report condition. Taken together, these findings challenge several prominent theories of consciousness and offer a new avenue for exploring the neural mechanisms supporting conscious processing.

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